1. Field of the Invention
The invention relates to desulfurizing coal, and more particularly to a process for removing sulfate, pyritic and especially organic sulfur from coal.
2. Discussion of Prior Art
There are three forms of sulfur in coal, pyritic, organic, and sulfate sulfur. Typically, the pyritic and organic sulfur contents are about the same in most Illinois Basin coals, while the sulfate sulfur content in most coals is very low. Present day commercial coal washing operations involve a separation of sulfur containing minerals as a function of their density. Inorganic sulfur compounds, mostly pyrites and marcasite, are more dense than the organic materials in coal. The density of a washing medium is adjusted so that the organic portion floats and the mineral components containing pyrite and marcasite sink to the bottom of the washing cell and are moved. This removes a portion of the pyritic sulfur, some of the sulfate sulfur and very, very little of the organic sulfur in coal.
A chemical cleaning process is needed to remove the organic sulfur from coal. Several methods of chemical cleaning of coal have been investigated. The Ledgemont Oxygen Leaching (LOL) Process uses 10-20 atm partial pressure oxygen, 130 degrees Celsius for 2 hours to remove up to 90% of the pyritic sulfur from pulverized coal. The organic sulfur is not affected. Under alkaline conditions 80-85% of the pyritic sulfur and 30-40% of the organic sulfur is removed. A known Oxydesulfurization Process is similar to the LOL Process but an iron-complexing agent such as oxalic acid or its salt is added to act as a promoter. Pyritic sulfur removal approaches 95% and organic sulfur removal approaches 35% for pulverized coals. This process typically employs alkaline solutions at 150 degrees Celsius and oxygen at 14 atmospheres to leach over 95% of the pyritic sulfur within 1 hour from certain coals. Organic sulfur removal varies from coal to coal but can approach 50%. In the oxidation by Nitrogen Oxides Process the active oxidizing agent is nitrogen dioxide. Pulverized coal is heated for 1 hour at 100 degrees Celsius under atmospheric pressure with a gas mixture of oxygen, nitrogen, nitrogen dioxide, and nitrogen oxide. The coal is then treated with hot caustic solution to remove the oxidized organic sulfur compounds. The pyritic sulfur and up to 40% of the organic sulfur can be removed.
None of the above processes have been developed commercially, due to the reaction requirements for the processes and the economics of implementing them.